1. Field of the Invention
This invention relates to electronic devices. In particular the invention relates to electronic devices employing the Gunn effect to produce coherent electronic oscillations.
2. Description of Related Art
Such devices have been known for a number of years, and typically comprise a quantity of a Gunn effect material, for example a material such as GaAs having two conduction bands of different curvatures, together with means for injecting electrons into the quantity. By application of an electric field of sufficient magnitude across the quantity, the electrons injected into the quantity may be excited from the lower to the upper conduction band. As the electrons within the upper conduction band will have a higher effective mass than those in the lower band, they will have a lower mobility than the electrons in the lower band. Thus, as these lower mobility electrons drift across the quantity, along the electric field gradient, they will form bunches amongst the higher mobility electrons, these bunches being known as domains. Since only one domain will be formed within the quantity at any one time, the output of the device will comprise a series of current pulses, whose frequency is dependent on the length of the quantity through which the domains drift, i.e. the transit region of the device.
Such a device suffers the disadvantage however that it is difficult to define the exact length of the transit region, due to the fact that it is difficult to control the region where the excitation of the electrons from the lower to the upper conduction band takes place, this imprecision causing noise in the output signal of the device. Furthermore the "dead space" within the quantity of Gunn effect material in which the energy of the electrons is insufficient to enable them to be excited into the upper conduction band causes parasitic resistance in the device. Attempts have been made to overcome these problems by using a material for the charge injection region which has a sufficiently large band gap offset to that of the Gunn material that the electrons injected into the quantity of Gunn material have nearly all the energy required to excite them into the upper conduction band at the injector material/Gunn material interface. Such a device suffers the problem however that there will be a relatively wide depletion region within the quantity of Gunn material adjacent to the injection region.